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ROUTING ON THE INTERNET COSC 6590 1 14-Aug-15. Routing Protocols  routers receive and forward packets  make decisions based on knowledge of topology.

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Presentation on theme: "ROUTING ON THE INTERNET COSC 6590 1 14-Aug-15. Routing Protocols  routers receive and forward packets  make decisions based on knowledge of topology."— Presentation transcript:


2 Routing Protocols  routers receive and forward packets  make decisions based on knowledge of topology and traffic/delay conditions  use dynamic routing algorithm  distinguish between:  routing information - about topology & delays  routing algorithm - that makes routing decisions based on information 2

3 Performance Criteria  used for selection of route  simplest is “minimum hop”  can be generalized as “least cost” 3

4 Example Packet Switched Network 4

5 Autonomous Systems (AS)  is a group of routers and networks managed by single organization  which exchange information via a common routing protocol  form a connected network  at least one path between any pair of nodes  except in times of failure 5

6 Interior and Exterior Router Protocols  interior router protocol (IRP)  passes routing information between routers within AS  can be tailored to specific applications  needs detailed model of network to function  may have more than one AS in internet  routing algorithms & tables may differ between them  routers need info on networks outside own AS  use an exterior router protocol (ERP) for this  supports summary information on AS reachability 6

7 Application of IRP and ERP 7 R1, R5: gateways

8 Approaches to Routing  Distance vector  Bellman-Ford  Routing Information Protocol ( RIP)  Interior Gateway Routing Protocol (IGRP, Cisco proprietary)  Link state  OPSF  OLSR protocol for MANETs 8

9 Distance Vector Routing  each node (router or host) exchange information with neighboring nodes  first generation routing algorithm for ARPANET  eg. used by Routing Information Protocol (RIP)  each node maintains vector of link costs for each directly attached network and distance and next-hop vectors for each destination  requires transmission of much info by routers  distance vector & estimated path costs  changes take long time to propagate 9

10 Link State Routing  designed to overcome drawbacks of distance-vector  each router determines link cost on each interface  advertises set of link costs to all other routers in topology  if link costs change, router advertises new values  each router constructs topology of entire configuration  can calculate shortest path to each dest  use to construct routing table with first hop to each dest  do not use distributed routing algorithm, but any suitable alg to determine shortest paths, eg. Dijkstra's algorithm  Open Shortest Path First (OSPF) is a link-state protocol 10

11 Exterior Routing  link-state and distance-vector not effective for exterior routing protocols  distance-vector  assumes routers share common distance metric  but different ASs may have different priorities & needs  but have no info on AS’s visited along route  link-state  different ASs may use different metrics and have different restrictions  flooding of link state information to all routers unmanageable 11

12 Path Vector for Exterior Routing  alternative path-vector routing protocol  provides info about which networks can be reached by a given router and ASs crossed to get there  does not include distance or cost estimate  hence dispenses with concept of routing metrics  have list of all ASs visited on a route  enables router to perform policy routing  eg. avoid path to avoid transiting particular AS  eg. link speed, capacity, tendency to become congested, and overall quality of operation, security  eg. minimizing number of transit ASs 12

13 Border Gateway Protocol (BGP)  developed for use with TCP/IP model  is preferred exterior routing protocol of the Internet  uses messages sent over TCP connections  current version is BGP-4 (RFC 1771, RFC 4271) 13

14 BGP Functional Procedures  neighbor acquisition: agree to exchange routing info regularly  send OPEN messages to each otehr over a TCP connection. Reply with a KEEP-ALIVE message.  neighbor reachability: to maintain relationship  periodically issue KEEP-ALIVE messages to each other  network reachability: to update database of routes  each router maintains a database of the networks that it can reach and the preferred route for reaching each network. When a change is made to this database, the router issues an UPDATE message that is broadcast to all other routers implementing BGP. 14

15 BGP Routing Information Exchange  within AS a router builds topology picture using an interior routing protocol  router issues UPDATE messages to other routers outside AS using BGP  these routers exchange info with other routers in other ASs  routers must then decide best routes for exterior routing 15

16 Open Shortest Path First (RFC 2328)  interior routing protocol of the Internet  replaced Routing Information Protocol (RIP)  uses link state routing algorithm  each router keeps list of state of local links to network  transmits update state info  little traffic as messages are small and not sent often  uses least cost based on user cost metric  topology stored as directed graph  vertices or nodes (router, transit or stub network)  edges (between routers or router to network) 16

17 Example OSPF AS 17

18 Directed Graph of AS 18

19 SPF Tree for Router 6 19

20 Reference  Data and Computer Communications, William Stallings, 8 th edition, section 19.2 20

21 Least Cost Algorithms  basis for routing decisions  can minimize hop with each link cost 1  or have link value inversely proportional to capacity  defines cost of path between two nodes as sum of costs of links traversed  in network of nodes connected by bi-directional links  where each link has a cost in each direction  for each pair of nodes, find path with least cost  link costs in different directions may be different  alternatives: Dijkstra or Bellman-Ford algorithms 21

22 Dijkstra’s Algorithm  finds shortest paths from given source node s to all other nodes  by developing paths in order of increasing path length  algorithm runs in stages (next slide)  each time adding node with next shortest path  algorithm terminates when all nodes processed by algorithm (in set T) 22

23 Dijkstra’s Algorithm Method  Step 1 [Initialization]  T = {s} Set of nodes so far incorporated  L(n) = w(s, n) for n ≠ s  initial path costs to neighboring nodes are simply link costs  Step 2 [Get Next Node]  find neighboring node not in T with least-cost path from s  incorporate node into T  also incorporate the edge that is incident on that node and a node in T that contributes to the path  Step 3 [Update Least-Cost Paths]  L(n) = min[L(n), L(x) + w(x, n)] for all n  T  f latter term is minimum, path from s to n is path from s to x concatenated with edge from x to n 23

24 Dijkstra’s Algorithm Example 24

25 Dijkstra’s Algorithm Example IterTL(2)PathL(3)PathL(4)PathL(5)PathL(6 ) Path 1{1}21–251-311–4  -  - 2{1,4}21–241-4-311–421-4–5  - 3{1, 2, 4}21–241-4-311–421-4–5  - 4{1, 2, 4, 5} 21–231-4-5–311–421-4–541-4-5–6 5{1, 2, 3, 4, 5} 21–231-4-5–311–421-4–541-4-5–6 6{1, 2, 3, 4, 5, 6} 21-231-4-5-311-421-4–541-4-5-6 25

26 Reference  Data and Computer Communications, William Stallings, 8 th edition, section 12.3.

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